1. Field of the Invention
The present invention relates to a diesel particulate filter apparatus to be incorporated into an exhaust system for purification treatment of exhaust gas discharged from a diesel engine.
2. Description of the Prior Art
In general, a diesel engine generates particulates such as carbon, soot and HC because of heterogeneous combustion wherein fuel sprayed into air elevated in temperature through adiabatic compression is combusted.
A diesel particulate filter made of, e.g., cordierite as the material thereof is known as a conventional filter for collecting particulates such as carbon, soot and HC from exhaust gas discharged from a diesel engine. A filter made using as the material thereof an inorganic fiber instead of cordierite is also known. In a diesel particulate filter apparatus having a filter body made using an inorganic fiber, the filter body is produced by laminating an inorganic fiber material and locally weaving an inorganic fiber material into the resulting laminate material to effect felting thereof. The filter body made of the inorganic fiber material can be miniaturized as compared with a honeycomb filter made of cordierite because the former enables particulates in exhaust gas discharged from a diesel engine to be trapped in the inner part of the filter body as well as on the surface thereof.
As shown in FIGS. 1 and 2, a diesel particulate filter apparatus wherein a filter is disposed in an exhaust gas flow path to filter exhaust gas for collecting particulates in exhaust gas was developed. Since a shutoff plate 33 for regulation of the exhaust gas flow path is disposed on the downstream side of the filter 31 in a cylindrical form, the diesel particulate filter apparatus is constructed in such a way that exhaust gas introduced into the filter 31 from the upstream side thereof is passed across the filter provided along the sidewall of a cylinder to be flowed into an exhaust path 32. Under control with a controller 34, electricity is flowed through a heater provided on the filter 31 to incinerate particulates collected with the filter.
In the particulate filter, a ceramic fiber laminate 25 having ceramic fibers randomly laminated and integrated together in a felty or papery form, and sandwiched between metal nets 27 as a heater is used as a filter material 23. The filter material 23 is bent in a wavy profile for enlargement of the collection surface area thereof, and then formed in a cylindrical form as a whole to produce the filter 31. As shown in FIGS. 3 and 4, an upstream seal cap 37 and a downstream seal cap 38 are used for sealing both ends of the filter 31. Thus, the filter 31 is formed. The seal cap 37 is provided with an opening 39 for allowing passage therethrough of exhaust gas.
Sealing between the filter 31 and the seal caps 37 and 38 is carried out according to the following procedure. Prior to sealing, a sealant is cast into the respective recesses 36 and 29 of the seal caps 37 and 38. A mixture prepared by adding 1 part of waterglass (in the form of a mixture of 1 part of waterglass admixed with 1 part of water) to 1 part of a material comprising 80 wt. % of an alumina powder and 20 wt. % of an alumina staple fiber (1 to 30 mm) is used as the sealant. Before the sealant is dried, the end portions of the filter 31 are fitted into the recesses of the seal caps 37 and 38, followed by heating thereof up to 350.degree. C. for solidification of the sealant.
Since the filter material 23 is elastic in the thickness-wise direction thereof and hence not necessarily uniformly settled in thickness because it is constructed in such a way that the ceramic fiber laminate is sandwiched between the wire nets 27. Further, the cross-sectional form of the filter 31 is wavy for enlargement of the collection surface area thereof. In sealing between the filter 31 and the seal caps 37 and 38, adhesion therebetween is sometimes unsatisfactory to cause leakage of exhaust gas though the former has the same wavy cross section as those of the latter. As a result, the diesel particulate filter apparatus involves a problem that particulates in exhaust Gas are discharged out of the apparatus without collection therewith of particulates.
On the other hand, Japanese Patent Laid-Open No. 108,820/1994 discloses a filter produced using a nonwoven fabric made of a ceramic fiber. End portions of the filter are put between and carried by metal sealing members, and supported by a case. In order to carry the end portions of the filter put between the metal sealing members, an areal pressure of about 10 Mpa is necessary. When the end portions of the filter are sandwiched between the sealing members under such a high areal pressure, the ceramic fiber constituting the nonwoven fabric is broken and the end portions of the filter come off.
Meanwhile, the applicant of the instant application developed a diesel particulate filter made of an inorganic filament, and previously filed a patent application directed thereto (Japanese Patent Application No. 114,131/1994). This diesel particulate filter is produced by horizontally irregularly orienting and laminating cut pieces of a heat-resistant inorganic filament cut to a predetermined length, needling the resulting laminate to effect vertical entanglement of the inorganic filament, superposing wire nets made of a heat-resistant metal on both the upper and lower surfaces of the resulting felty material, and fastening them from both surfaces thereof with a heat-resistant yarn. Herein, the above-mentioned inorganic filament is, for example, a fiber made of a material selected from among Si--Ti--C--O, Si--C--O, Si--N, SiO and metals, and having the surface thereof coated with at least one of SiC, Al and Al.sub.2 O.sub.3.
When a diesel particulate filter apparatus for collection therewith of particulates such as carbon and smoke contained in exhaust gas is produced using the filter produced by laminating a heat-resistant ceramic fiber such as an SiC fiber or an Si--Ti--C--O fiber, however, pieces of the fiber material must be alternately woven into one another in the step of laminating the ceramic fiber to create a difficulty in forming the filter through laminating thereof because the ceramic fiber material is liable to break. Specifically, when a filter body is produced using the ceramic fiber, the ceramic fiber material constituting the filter body is brittle in itself and hence weak against vibrations and impacts to involve problems such as a fear of breakage and a high production cost. Further, the ceramic fiber such as an SiC fiber in particular has a high Young's modulus of 430 GPa to be liable to break. In the step of producing the filter body by felting the ceramic fiber and in the step of assembling the ceramic fiber material-based diesel particulate filter apparatus, therefore, the ceramic fiber is broken into fine pieces and separated during bending thereof for forming the filter as well as assembly of the apparatus to create a difficulty in weaving pieces of the ceramic fiber material into one another for forming the filter body, and the separated fine pieces of the fiber are scattered to present a problem of deterioration of working environment.